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Technical ceramics in practice – Ceramic screws, ceramic bearings and hybrid bearings
Technical ceramics are ceramics used in specialized technical applications. Due to their special properties, such as resistance to high temperatures and chemicals, they are often the better choice in certain applications than components made of metal, for example. This article shows some of the possible applications and discusses the material composition and production of technical ceramics.
What are technical ceramics?
Technical ceramics, also called specialty ceramics or high-performance ceramics, are optimized for technical applications. They differs from conventional ceramics, e.g., by purity, kiln processes, and tolerated grain size.
Standardization of technical ceramics
Technical ceramics are subject to various standards. For example, the following standards exist for oxide ceramics:
- DIN EN 60672: Defines the group classification, terms, test procedures. Minimum requirements for properties, such as resistance To bending, are also specified.
- DIN 40680: Defines general tolerances for ceramic components in the electrical engineering field.
- DIN EN 14232: Deals with high-performance ceramics and lists terms, incl. definitions.
- ISO 15165: Contains a high-performance ceramic classification system.
Certain test methods are also standardized. DIN EN 725 contains, for example, specifications for impurities and density, among other things for high-performance ceramic powders.
Materials for technical ceramics
Ceramic is a generic term for various inorganic, non-metallic materials. As a rule, a mixture of ceramic powder, organic binder and liquid are used to generate a raw compound, which must then be cured (e.g. in a sintering process at high temperatures). Ceramics can be divided into three main categories: ceramic earthenware, sintered ceramics, and special-purpose ceramic compounds. Technical ceramics are one of the special-purpose ceramic compounds. In general, technical ceramics can be subdivided into oxidic and non-oxidic ceramics, wherein oxidic ceramics such as aluminum oxide are used more frequently. Oxidic ceramics consist of metal oxides and are characterized by chemical stability, strength and electrical insulating capability. Non-oxidic ceramics exhibit high wear resistance (resistance to abrasion), possibly better thermal conductivity, and mechanical load resistance. They are further divided into:
- Nitride ceramics: Nitride ceramics contain nitrogen. Silicon nitride, for example, has high thermal shock resistance and high wear resistance. Boritride has good lubricity.
- Carbide ceramics: Carbide ceramics contain carbon. They are particularly hard, with boron carbide as one of the hardest materials. Silicon carbide has a high melting point (approx. 2700°C) and is chemically stable.
- Silica ceramics: Silica ceramics are based on silicon dioxide. Examples include porcelain and steatite. Steatite has good dielectric properties and is often used as an insulator in electrical engineering.
The following table provides an overview of the classification of the different types of ceramics:
| Ceramic | ||||||||||
|---|---|---|---|---|---|---|---|---|---|---|
| Ceramic stonewear | Ceramic sintered material | Specialty ceramic compounds, e.g. high-temperature and electrotechnical specialty compounds | ||||||||
| Construction ceramics | Fireproof ceramics | Miscellaneous stonewear | Stoneware | Porcelain | Technical ceramics (silicate ceramics / oxide ceramics / non-oxide ceramics) | |||||
| Bricks, roof tiles, etc. | Schamoth stones, magnesite, etc. | Stoneware | Pottery | Coarse stoneware | Porcelain stoneware | Hard porcelain | Soft porcelain | Traditional technical ceramics | High-performance ceramics | |
| Tableware, etc. Tiles | Flower pots, terracotta, etc. | Clinkers, tiles, sewer pipes, etc. | Tiles, dishes, sanitary items, chemical equipment, etc. | Household and decorative dishes | preferred for decorative plastics | Chemical porcelain, fire-resistant ceramics, insulators | Functional ceramics | Structural and engineering ceramics | ||
| Sensor and protective ceramics, bio- and medical ceramics, electro-ceramic, cutting ceramics | Mechanically loaded parts with high hardness and wear resistance such as: seals, bearings, bushings, structural components | |||||||||
Manufacturing processes for technical ceramics
There are various manufacturing processes for technical ceramics. Hot isostatic pressing (HIP), also called high-pressure sintering, is used to manufacture ceramics with very high density and low porosity.
However, it is also possible to print components made of technical ceramics directly on a printer. The LCM process (lithography-based ceramic manufacturing) for example uses a UV-sensitive monomer and a ceramic powder as the raw material. The LDM process (liquid deposition modeling) involves moistening and compacting ceramic raw material, followed by applying the latter layer by layer using a printer.
Properties of technical ceramics
The properties of technical ceramics make them the preferred choice in specialized applications. Due to their high temperature resistance, they are suited for use in high temperature applications, such as power generation. They do not lose their structural integrity when heated. Technical ceramics are also chemically more resistant than other materials, since they can be chemically inert.
However, the high hardness and density of the ceramic is accompanied by reduced strength at rupture, which must be taken into account in design:
- sharp edges, corners and notches should be avoided or at least minimized. These can lead to cracks and stresses. Rounded edges can for example be used instead.
- Excessively tight fits should be avoided, as they also lead to cracks.
- When drilling, a sufficiently large radius must be used to avoid stresses.
- Technical ceramics insulate electrical energy very effectively. Their use may have to be avoided if this is not desired.
The following tables provide an overview of various properties of technical ceramics, in particular aluminum oxides, along with a comparison to other materials:
| Materials | Color | Properties | ||
|---|---|---|---|---|
| Safe ambient temperature (°C) | Volume-specific resistance(Ω * cm) | Flexural strength Mpa | ||
| Aluminum oxide 92 / Al2O3 92% | white | ~ 1200 | > 1014 | 240~340 |
| Al2O3 / aluminum oxide 96 / Al2O3 96% | white | ~ 1300 | > 1014 | 300 |
| Al2O3 / aluminum oxide 99 / Al2O3 99.7% | natural colors | ~ 1500 | > 1015 | 340 |
| Aluminum oxide 99.5 | white | ~ 1200 | < 1014 | 490 |
| Steatit / SiO 2, MgO | white | ~ 1000 | > 1014 | 120 |
| Machinable ceramics / SiO2, MgO | natural colors | ~ 1000 | > 1016 | 94 |
| Properties | Unit | Al2O3 / aluminum oxide 99.5 |
|---|---|---|
| Water absorption ratio | % | |
| Density | g/cm3 | 3.9 |
| Heat resistance | ℃ | 1000 ~ 1200 |
| Compressive strength | kN/cm2 | 363 |
| Flexural strength | kN/cm2 | 49 |
| Linear coefficient of thermal expansion | - | 8.0x10-6 (25~700 ℃) |
| Thermal conductivity | W/(m x ℃) | 31.4 (20 ℃) 16.0 (300 ℃) |
| Specific volume resistance | Ω x cm | > 1014 (20 ℃) > 1014 (300℃) |
| Dielectric constant | 1 MHz | 9.8 |
| Insulation resistance | kV/mm | 10 |
| Physical properties of Al2O3 (representative reference values) |
| Stainless steel 1.4301/X5CrNi18-10 |
Centering pin (KCF) (stainless steel with 5~10μm coating made of aluminum oxide as an insulating layer) |
Ceramic Al2O3 | Nylon | Bakelite (paper-based) |
Bakelite (fabric-based) |
|
|---|---|---|---|---|---|---|
| Natural resistance (Ω) | 72x10-6 | 2x108 | 1014 | 5x1012 | 1010 | 1012 |
| Breakdown voltage (V) | - | 150 | 104 | 1.9x104 | - | - |
| Tear strength (MPa) | 520 | 421 | - | 88 | 80 | 100 |
| Expansion (%) | 40 | 10 | - | 50 | 2 | 2 |
| Flexural strength (MPa) | - | - | 350 | 103 | 180 | 160 |
| Vickers hardness (HV) | 200 | at the tip 1300 inside 200 |
1400 | - | - | - |
| Insulating properties | ❌ | good | excellent | excellent | excellent | excellent |
| Heat resistance | good | good | excellent | ❌ | questionable | questionable |
| Machinability | good | good | ❌ | good | good | good |
Use of technical ceramics
Technical ceramics are usually used for specialized requirements. As a rule, ceramic materials are corrosion and temperature-resistant, electrically insulating and at the same time relatively light, compression-resistant and wear-resistant. If the increased brittleness of the ceramic is taken into account in the design, the mechanical strength of high-performance ceramics allows not only weight savings and use at higher temperatures, but also lower heat generation, operating noise reduction and longer service life in bearings. Standard parts such as screws and washers are also available in technical ceramics.
Ceramic bearings and hybrid bearings
Ceramic bearings are chemically resistant and are suited for dry-running applications without lubrication. Thanks to the excellent rolling properties of ceramic roller bodies, they are exceedingly suited for high rotational speeds. Fully-ceramic bearings do not rust and cannot be influenced by magnetic fields, but are susceptible to shock and tensile stresses. Applications include, for example, cleaning equipment, electroplating equipment, and etching equipment.
Ceramic bearings are available as fully-ceramic and hybrid bearings. Rolling elements made of high-performance ceramics and also bearing rings made of rolling bearing steel are installed in hybrid bearings. As a result, a hybrid bearing combines the benefits of both materials, thus improving performance. Hybrid bearings are suited for use at high rotational speeds and in difficult lubrication conditions. Ceramic and hybrid bearings are also recommended at high temperatures of up to 1000°C, in corrosion-inducing environments, in lightweight construction (up to 60% lighter than steel bearings) and when electrical insulation is required. However, when ceramic bearings are used, it is important to note that these expand to a lesser extent than, for example, steel bearings. If designs that are exposed to high temperature influences are specified for the use of ceramic bearings, these cannot be readily replaced by steel bearings.
Ceramic screws
In addition to the aforementioned properties for ceramics in general, ceramic screws are also particularly compelling due to the following properties: electrically insulating, non-magnetic and light-weight, which differentiates them from metal screws. They can be used, for example, in electronic assemblies or in applications for which magnetic interference is undesirable (e.g. also electronics, medical equipment).
Ceramic screws are available in the following variants, for example:
- Zirconia screws: very hard, wear-resistant, thermal shock resistant
- Aluminum oxide screws: very hard, temperature resistant
- Silicon nitride screws: especially light-weight due to low density
Installation instructions
The following notes should be observed to ensure that the ceramic component is incorporated as-best-as-possible in the design:
- Components made of ceramic are very susceptible to shock; special caution should therefore be exercised during installation.
- Ceramic screws should always be tightened with torque. They are more fragile than metal screws, so the torque should be lower, e.g. 0.04 for M3, 0.05 for M4, 0.30 for M8 and 0.50 for M10.
- Washers are recommended for better load distribution.
- Alignment is particularly important for roller bearings: Unequal loads can lead to premature failure.